Scanning Tunneling Microscopy Break Junction Research Articles

Ultra-highly conductive optoelectronic modulated single-molecule nickel bis(dithiolene) junctions with strong molecule-electrode coupling

Strong molecule-electrode coupling originating from orbit hybridization between gold and the delocalized molecular wires in single-molecule junctions facilitates facile transport towards smart molecular devices. In this paper, we report ultra-highly conductive single-molecule circuits based on highly delocalized nickel bis(dithiolene) (NiS4) molecular junctions using scanning tunneling microscope break junction technique. Single-molecule charge transport measurement of both NiS4 reveals they exhibits high conductance of 10−1.49G0 and 10−1.51G0, respectively. Moreover, under intervention of high bias voltage the molecular conductance could be further improved to approximately 10−1.00G0, the highest value reported to date with similar molecular lengths. Theoretical calculations suggest that the strong hybridization of the π-channels and the gold electrodes in both junctions exists and it further extends from molecule-electrode interfaces to metal electrodes as visualized by the isosurface plots of the transmitting eigenstate, which lead to not only a distinct energy shift of the dominated LUMO peaks toward Fermi level, but also broad peaks in the LUMO resonance in the transmission functions. In addition, the both molecular junctions show remarkable photoconductance of approximately 10−1.00G0 under resonant light excitation, due to possible exciton binding in these junctions. Interestingly, the conductance switching of both molecular junctions under optoelectronic modulation is highly reversible, forming a multi-stimulus responsive molecular switch. This work not only provides a building block for fabricating highly conducting molecular wires with strong molecule-electrode coupling, but also lays a foundation for designing optoelectronic modulated functional molecule-scale devices.

A Helicene-Based Single-Molecule Inductor and Capacitor with Frequency-Dependent Charge-Transport Pathways.

Despite extensive studies has been explored on single-molecule switches and rectifiers, the design of single-molecule inductors has not been explored due to the experimental challenges in the investigation of frequency-dependent charge transport at the single-molecule scale. In this study, we synthesized a helicene-based helical molecular wire and carried out meticulous single-molecule conductance measurements, combined with current-voltage (IV) studies with varying frequencies using the scanning tunneling microscope break junction (STM-BJ) technique. Our results reveal the formation of a single-molecule junction and highlight the unique behavior of the molecular wire in response to different alternating current (AC) varying frequencies. The transport of charges occurs selectively either through the coiled backbone of the conjugated helical structure or vertically via π-π stacking, depending on the frequency of the applied AC. Notably, our investigation demonstrates the functionality of the wire as an inductor at low frequencies, and a capacitor at high frequencies. This work lays the foundation for a systematic approach to designing, fabricating, and implementing single-molecule logic devices such as inductors and wave filters.

Towards Stable Helical Structures with Enhanced Molecular Conductance by Strengthening Through-Space Conjugation.

Developing long-chain molecules with stable helical structures is of significant importance for understanding and modulating the properties and functions of helical biological macromolecules, but challenging. In this work, an effective and facile approach to stabilize folded helical structures by strengthening through-space conjugation is proposed, using new ortho-hexaphenylene (o-HP) derivatives as models. The structure-activity relationship between the through-space conjugation and charge-transport behavior of the prepared folded helical o-HP derivatives is experimentally and theoretically investigated. It is demonstrated that the through-space conjugation within o-HP derivatives can be strengthened by introducing electron-withdrawing pyridine and pyrazine rings, which can effectively stabilize the helical structures of o-HP derivatives. Moreover, scanning tunneling microscopy-break junction measurements reveal that the stable regular helical structures of o-HP derivatives open-up dominant through-space charge-transport pathways, and the single-molecule conductance is enhanced by more than 70 % by strengthening through-space conjugation with pyridine and pyrazine. However, the through-bond charge transport pathways contribute much less to the conductance of o-HP derivatives. These results not only provide a new method for exploring stable helical molecules, but also provide a stepping stone for deciphering and modulating the charge-transport behavior of helical systems at the single-molecule level.

Label-Free Single-Molecule Electrical Sensor for Ultrasensitive and Selective Detection of Iodide Ions in Human Urine.

Herein, a label-free single-molecule electrical sensor was first proposed for the ultrasensitive and selective detection of iodide ions in human urine. Single-molecule conductance measurements in different halogen ion solutions via scanning tunneling microscopy break junction (STM-BJ) clearly revealed that I- ions strongly affect the stability and displacement distance (Δz) distribution of molecular junctions. Theoretical calculations prove that the specific adsorption of I- ions modifies the surface properties and weakens the molecular adsorption. Furthermore, the average conductance peak area versus the logarithm of the I- ion concentration has a very good linear relationship in the range of 5 × 10-6 to 5 × 10-10 M, with a correlation coefficient of 0.99. This quantitative analysis remains valid in the presence of interfering ions of SO42-, ClO4-, Br-, and Cl- as well as interfering molecules of ascorbic acid, uric acid, dopamine, and cysteine. A cross-comparison of the human urine detection results of this single-molecule electrical sensor with those of the clinical method of As3+-Ce4+ catalytic spectrophotometry revealed an average difference of 0.9%, which decreased the detection time of 2 h with the traditional method to approximately 15 min. This work proves the promising practical potential of the single-molecule electrical technique for relevant clinical analysis.

Single‐Molecule Conductance of Staffanes

We report the first conductance measurements of [n]staffane oligomers in single‐molecule junctions. Our studies reveal two quantum transport characteristics unique to staffanes that emerge from their strained bicyclic structure. First, though staffanes are composed of weakly conjugated C‐C σ‐bonds, staffanes carry a shallower conductance decay value (β = 0.84 ± 0.02 n‐1) than alkane chain analogs (β = 0.96 ± 0.03 n‐1) when measured with the scanning tunneling microscopy break junction (STM‐BJ) technique. Staffanes are more conductive than other σ‐bonded organic backbones in the literature on a per atom basis. Density functional theory calculations suggest staffane backbones are effective conduits for charge transport because their significant bicyclic ring strain destabilizes the HOMO‐2 energy, aligning it more closely with the Fermi energy as oligomer order increases. Second, the monostaffane is significantly lower conducting than expected. DFT calculations suggest that short monostaffanes sterically enforce insulating gauche interelectrode orientations over syn orientations. Meanwhile, [2‐5]staffane wires may accommodate axial mechanical strain by “rod‐bending”. These findings show for the first time how bicyclic ring strain can enhance charge transmission in saturated molecular wires. These studies showcase the STM‐BJ technique as a valuable tool for uncovering the stereoelectronic proclivities of molecules at material interfaces.

Single-Molecule Conductance of Staffanes.

We report the first conductance measurements of [n]staffane (bicyclopentane) oligomers in single-molecule junctions. Our studies reveal two quantum transport characteristics unique to staffanes that emerge from their strained bicyclic structure. First, though staffanes are composed of weakly conjugated C-C σ-bonds, staffanes carry a shallower conductance decay value (β=0.84±0.02 n-1) than alkane chain analogs (β=0.96±0.03 n-1) when measured with the scanning tunneling microscopy break junction (STM-BJ) technique. Staffanes are thus more conductive than other σ-bonded organic backbones reported in the literature on a per atom basis. Density functional theory (DFT) calculations suggest staffane backbones are more effective conduits for charge transport because their significant bicyclic ring strain destabilizes the HOMO-2 energy, aligning it more closely with the Fermi energy of gold electrodes as oligomer order increases. Second, the monostaffane is significantly lower conducting than expected. DFT calculations suggest that short monostaffanes sterically enforce insulating gauche interelectrode orientations over syn orientations; these steric effects are alleviated in longer staffanes. Moreover, we find that [2-5]staffane wires may accommodate axial mechanical strain by "rod-bending". These findings show for the first time how bicyclic ring strain can enhance charge transmission in saturated molecular wires. These studies showcase the STM-BJ technique as a valuable tool for uncovering the stereoelectronic proclivities of molecules at material interfaces.

Covalent Au-C Contact Formation and C-C Homocoupling Reaction from Organotin Compounds in Single-Molecule Junctions.

Formation of new chemical species has been achieved under an electric field by the use of the scanning tunneling microscope break junction technique, yet simultaneous implementation of catalytic reactions both at the organic/metal interface and in the bulk solution remains a challenging task. Herein, we show that n-butyl-substituted organotin-terminated benzene undergoes both an efficient cleavage of the terminal tributyltin group to form a covalent Au-C bond and a homocoupling reaction to yield biphenyl product when subjected to an electric field in the vicinity to Au electrodes. By using ex situ characterization of high-performance liquid chromatography with an UV-vis detector, we demonstrate that the homocoupling reaction can occur with high efficiency under an extremely low tip bias voltage of ∼5 mV. Additionally, we show that the efficiency of the homocoupling reaction varies significantly in different solvents; the choice of the solvent proves to be one of the methods for modulating this reaction. By synthesizing and testing varied molecular backbone structures, we show that an extended biphenyl backbone undergoes homocoupling to form a quarterphenylene backbone, and the C-C coupling reactions are prohibited when additional aurophilic or bulky chemical groups that exhibit a steric blockage are introduced.

A Helicene‐Based Single‐Molecule Inductor and Capacitor with Frequency‐Dependent Charge‐Transport Pathways

Despite extensive studies has been explored on single‐molecule switches and rectifiers, the design of single‐molecule inductors has not been explored due to the experimental challenges in the investigation of frequency‐dependent charge transport at the single‐molecule scale. In this study, we synthesized a helicene‐based helical molecular wire and carried out meticulous single‐molecule conductance measurements, combined with current‐voltage (IV) studies with varying frequencies using the scanning tunneling microscope break junction (STM‐BJ) technique. Our results reveal the formation of a single‐molecule junction and highlight the unique behavior of the molecular wire in response to different alternating current (AC) varying frequencies. The transport of charges occurs selectively either through the coiled backbone of the conjugated helical structure or vertically via π‐π stacking, depending on the frequency of the applied AC. Notably, our investigation demonstrates the functionality of the wire as an inductor at low frequencies, and a capacitor at high frequencies. This work lays the foundation for a systematic approach to designing, fabricating, and implementing single‐molecule logic devices such as inductors and wave filters.

In Situ Detection of Interfacial Ions at the Single-Bond Level.

Detecting the ionic state at the solid-liquid interface is essential to reveal the various chemical and physical processes that occur at the interface. In this study, the adsorption states of the highly electronegative ions F- and OH- at the solid-liquid interface are detected by using the scanning tunneling microscopy break junction technique. With the active hydrogen atom of the amino group as a probe, the formed ionic hydrogen bonds are successfully detected, thereby enabling in situ monitoring of the ionic state at the solid-liquid interface. Through noise power spectral density analysis and theoretical simulations, we reveal the mechanism by which ionic hydrogen bonds at the interface affect the charge transport properties. In addition, we discover that the ionic state at the solid-liquid interface can be effectively manipulated by electric fields. Under high electric fields, the concentration of the anion near the electrode is higher, and the proportion of hydrogen bonds formed is greater than that under low electric fields. This study of the interfacial ionic state at the single-bond level provides guidance for the design of high-performance materials for energy conversion and environmental purification.

The Conductance and Thermopower Behavior of Pendent Trans-Coordinated Palladium(II) Complexes in Single-Molecule Junctions.

The present work provides insight into the effect of connectivity within isomeric 1,2-bis(2-pyridylethynyl)benzene (bpb) palladium complexes on their electron transmission properties within gold|single-molecule|gold junctions. The ligands 2,2'-((4,5-bis(hexyloxy)-1,2-phenylene)bis(ethyne-2,1-diyl))bis(4-(methylthio)pyridine) (Lm ) and 6,6'-((4,5-bis(hexyloxy)-1,2-phenylene)bis(ethyne-2,1-diyl))bis(3-(methylthio)pyridine) (Lp ) were synthesized and coordinated with PdCl2 to give the trans-Pd(Lmorp )Cl2 complexes. X-ray photoelectron spectroscopy (XPS) measurements shed light on the contacting modes of the molecules in the junctions. A combination of scanning tunneling microscopy-break junction (STM-BJ) measurements and density functional theory (DFT) calculations demonstrate that the typical lower conductance of meta- compared with para-connected isomers in a molecular junction was suppressed upon metal coordination. Simultaneously there was a modest increase in both conductance and Seebeck coefficient due to the contraction of the HOMO-LUMO gap upon metal coordination. It is shown that the low Seebeck coefficient is primarily a consequence of how the resonances shift relative to the Fermi energy.

Impact of Solvent Electrostatic Environment on Molecular Junctions Probed via Electrochemical Impedance Spectroscopy.

The electrostatic environment around nanoscale molecular junctions modulates charge transport; solvents alter this environment. Methods to directly probe solvent effects require correlating measurements of the local electrostatic environment with charge transport across the metal-molecule-metal junction. Here, we measure the conductance and current-voltage characteristics of molecular wires using a scanning tunneling microscope-break junction (STM-BJ) setup in two commonly used solvents. Our results show that the solvent environment induces shifts in molecular conductance, which we quantify, but more importantly we find that the solvent also impacts the magnitude of current rectification in molecular junctions. By incorporating electrochemical impedance spectroscopy into the STM-BJ setup, we measure the capacitance of the dipole layer formed at the metal-solvent interface and show that rectification can be correlated with solvent capacitance. These results provide a method of quantifying the impact of the solvent environment and a path toward improved environmental control of molecular devices.

Interfacial Stereoelectronic Effect Induced by Anchoring Orientation.

Heterogeneous interfaces in most devices play a key role in the material performance. Exploring the atomic structure and electronic properties of metal-molecule interfaces is critical for various potential applications, such as surface sensing, molecular recognition, and molecular electronic devices. This study unveils a ubiquitous interfacial stereoelectronic effect in conjugated molecular junctions by combining first-principles simulation and scanning tunneling microscopy break junction technology. Single-molecule junctions with same-side interfacial anchoring (cis configuration) exhibit higher conductance than those with opposite-side interfacial anchoring (trans configuration). The cis and trans configurations can undergo reversible conversions, resulting in a conductance switching. The stability of these configurations can be adjusted by an electric field, achieving precise regulation of conductance states. Our findings provide important insights for designing high-quality materials and enhancing the device performance.

A Systematic Study of Methyl Carbodithioate Esters as Effective Gold Contact Groups for Single‐Molecule Electronics

AbstractThere are several binding groups used within molecular electronics for anchoring molecules to metal electrodes (e.g., R−SMe, R−NH2, R−CS2−, R−S−). However, some anchoring groups that bind strongly to electrodes have poor/unknown stability, some have weak electrode coupling, while for some their binding motifs are not well defined. Further binding groups are required to aid molecular design and to achieve a suitable balance in performance across a range of properties. We present an in‐depth investigation into the use of carbodithioate esters as contact groups for single‐molecule conductance measurements, using scanning tunnelling microscopy break junction measurements (STM‐BJ) and detailed surface spectroscopic analysis. We demonstrate that the methyl carbodithioate ester acts as an effective contact for gold electrodes in STM‐BJ measurements. Surface enhanced Raman measurements demonstrate that the C=S functionality remains intact when adsorbed on to gold nanoparticles. A gold(I) complex was also synthesised showing a stable C=S→AuI interaction from the ester. Comparison with a benzyl thiomethyl ether demonstrates that the C=S moiety significantly contributes to charge transport in single‐molecule junctions. The overall performance of the CS2Me group demonstrates it should be used more extensively and has strong potential for the fabrication of larger area devices with long‐term stability.

A Systematic Study of Methyl Carbodithioate Esters as Effective Gold Contact Groups for Single-Molecule Electronics.

There are several binding groups used within molecular electronics for anchoring molecules to metal electrodes (e.g., R-SMe, R-NH2, R-CS2 -, R-S-). However, some anchoring groups that bind strongly to electrodes have poor/unknown stability, some have weak electrode coupling, while for some their binding motifs are not well defined. Further binding groups are required to aid molecular design and to achieve a suitable balance in performance across a range of properties. We present an in-depth investigation into the use of carbodithioate esters as contact groups for single-molecule conductance measurements, using scanning tunnelling microscopy break junction measurements (STM-BJ) and detailed surface spectroscopic analysis. We demonstrate that the methyl carbodithioate ester acts as an effective contact for gold electrodes in STM-BJ measurements. Surface enhanced Raman measurements demonstrate that the C=S functionality remains intact when adsorbed on to gold nanoparticles. A gold(I) complex was also synthesised showing a stable C=S→AuI interaction from the ester. Comparison with a benzyl thiomethyl ether demonstrates that the C=S moiety significantly contributes to charge transport in single-molecule junctions. The overall performance of the CS2Me group demonstrates it should be used more extensively and has strong potential for the fabrication of larger area devices with long-term stability.

Synthesis, characterization, and electronic transport properties of o-carborane-bridged thiophene derivatives

Here, α- or β-thiophene-substituted derivatives bridged by an o-carborane group were synthesized. Scanning tunneling microscope break junction techniques have revealed that the thiophene anchoring groups form a single-molecule junction with an Au electrode. Flicker noise analysis suggests the existence of through-space transmission pathways between the thiophene units. Furthermore, the electronic coupling of the β-thiophene substituted molecule with the electrode is found to be more stable, resulting in a 70 % increase in conductance compared to the α-isomer counterpart.These findings offer valuable insights for the design and development of o-carborane-based molecules, underscoring the significance of molecular backbones and substitution patterns on their electrical transport properties.

Conductance Quantization in 2D Semi-Metallic Transition Metal Dichalcogenides.

Conductance quantization of 2D materials is significant for understanding the charge transport at the atomic scale, which provides a platform to manipulate the quantum states, showing promising applications for nanoelectronics and memristors. However, the conventional methods for investigating conductance quantization are only applicable to materials consisting of one element, such as metal and graphene. The experimental observation of conductance quantization in transition metal dichalcogenides (TMDCs) with complex compositions and structures remains a challenge. To address this issue, an approach is proposed to characterize the charge transport across a single atom in TMDCs by integrating in situ synthesized 1T'-WTe2 electrodes with scanning tunneling microscope break junction (STM-BJ) technique. The quantized conductance of 1T'-WTe2 is measured for the first time, and the quantum states can be modulated by stretching speed and solvent. Combined with theoretical calculations, the evolution of quantized and corresponding configurations during the break junction process is demonstrated. This work provides a facile and reliable avenue to characterize and modulate conductance quantization of 2D materials, intensively expanding the research scope of quantum effects in diverse materials.

Effects of Connectivity Isomerization on Electron Transport Through Thiophene Heterocyclic Molecular Junction.

Connectivity isomerization of the same aromatic molecular core with different substitution positions profoundly affects electron transport pathways and single-molecule conductance. Herein, we designed and synthesized all connectivity isomers of a thiophene (TP) aromatic ring substituted by two dihydrobenzo[b]thiophene (BT) groups with ethynyl spacers (m,n-TP-BT, (m,n = 2,3; 2,4; 2,5; 3,4)), to systematically probe how connectivity contributes to single-molecule conductance. Single-molecule conductance measurements using a scanning tunneling microscopy break junction (STM-BJ) technique show ∼12-fold change in conductance values, which follow an order of 10-4.83 G0 (2,4-TP-BT) < 10-4.78 G0 (3,4-TP-BT) < 10-4.06 G0 (2,3-TP-BT) < 10-3.75 G0 (2,5-TP-BT). Electronic structure analysis and theoretical simulations show that the connectivity isomerization significantly changes electron delocalization and HOMO-LUMO energy gaps. Moreover, the connectivity-dependent molecular structures lead to different quantum interference (QI) effects in electron transport, e.g., a strong destructive QI near E = EF leads the smallest conductance value for 2,4-TP-BT. This work proves a clear relationship between the connectivity isomerization and single-molecule conductance of thiophene heterocyclic molecular junctions for the future design of molecular devices.

Unveiling the Properties of Sulfhydryl Groups in a Single-Molecule Junction.

The metal-thiol interface is ubiquitous in nanotechnology and surface chemistry. It is not only used to construct nanocomposites but also plays a decisive role in the properties of these materials. When organothiol molecules bind to the gold surface, there is still controversy over whether sulfhydryl groups can form disulfide bonds and whether these disulfide bonds can remain stable on the gold surface. Here, we investigate the intrinsic properties of sulfhydryl groups on the gold surface at the single-molecule level using a scanning tunneling microscope break junction technique. Our findings indicate that sulfhydryl groups can react with each other to form disulfide bonds on the gold surface, and the electric field can promote the sulfhydryl coupling reaction. In addition to these findings, ultraviolet irradiation is used to effectively regulate the coupling between sulfhydryl groups, leading to the formation and cleavage of disulfide bonds. These results unveil the intrinsic properties of sulfhydryl groups on the gold surface, therefore facilitating the accurate construction of broad nanocomposites with the desired functionalities.

Intramolecular London Dispersion Interactions in Single-Molecule Junctions.

This work shows the first example of using intramolecular London dispersion interactions to control molecular geometry and quantum transport in single-molecule junctions. Flexible σ-bonded molecular junctions typically occupy straight-chain geometries due to steric effects. Here, we synthesize a series of thiomethyl-terminated oligo(dimethylsilmethylene)s that bear [CH2-Si(CH3)2]n repeat units, where all backbone dihedral states are sterically equivalent. Scanning tunneling microscopy break-junction (STM-BJ) measurements and theoretical calculations indicate that in the absence of a strong steric bias concerted intramolecular London dispersion interactions staple the carbosilane backbone into coiled conformations that remain intact even as the junction is stretched to its breakpoint. As these kinked conformations are highly resistive to electronic transport, we observe record-high conductance decay values on an experimental junction length basis (β = 1.86 ± 0.12 Å-1). These studies reveal the potential of using intramolecular London dispersion interactions to design single-molecule electronics.

Benchmarking break-junction techniques: electric and thermoelectric characterization of naphthalenophanes.

Break-junction techniques provide the possibility to study electric and thermoelectric properties of single-molecule junctions in great detail. These techniques rely on the same principle of controllably breaking metallic contacts in order to create single-molecule junctions, whilst keeping track of the junction's conductance. Here, we compare results from mechanically controllable break junction (MCBJ) and scanning tunneling microscope (STM) methods, while characterizing conductance properties of the same novel mechanosensitive para- and meta-connected naphtalenophane compounds. In addition, thermopower measurements are carried out for both compounds using the STM break junction (STM-BJ) technique. For the conductance experiments, the same data processing using a clustering analysis is performed. We obtain to a large extent similar results for both methods, although values of conductance and stretching lengths for the STM-BJ technique are slightly larger in comparison with the MCBJ. STM-BJ thermopower experiments show similar Seebeck coefficients for both compounds. An increase in the Seebeck coefficient is revealed, whilst the conductance decreases, after which it saturates at around 10 μV K-1. This phenomenon is studied theoretically using a tight binding model. It shows that changes of molecule-electrode electronic couplings combined with shifts of the resonance energies explain the correlated behavior of conductance and Seebeck coefficient.